Optical communications are evolving as the chosen technique for data and voice communications. Currently most OICs in the optical network are passive components that are discretely packaged, providing a singular functionality such as power splitting an optical signal into several signals (1×N), creating (N×M) switches for optical signals, equalizing or attenuating signals, wavelength demultiplexing via arrayed waveguide gratings, or adding and dropping selected wavelengths into the optical path (Optical Add-Drop Multiplexing). Higher levels of integration may combine several of these functions on a single OIC chip. In addition the hybrid integration of active dies such as lasers, modulators, and photodetectors has been accomplished and is gaining in popularity as more mature manufacturing methods are developed.
Although the technology is maturing rapidly, optical integrated circuits are still an order of magnitude larger than their electrical counterparts because of the large bend radius, large core size, and limited mode confinement of current, “low delta-n” (low refractive index differential) planar optical circuits. For example, planar waveguide cores that have the same mode field diameter of current telecommunications fiber can cause an 8×8 optical switch with power equalization to consume an entire 100 mm wafer. The reason large core size and low index contrast between the core and cladding are presently used in the current generation of OICs is to achieve a mode match between the current single mode optical fiber used in the networks and the OIC. Such mode matching achieves low coupling loss between the OIC and the optical fibers that connect it to the rest of the network. However, only limited density per wafer can be achieved in such OICs. Consequently, there is a desire to increase the index contrast to allow better utilization of wafer surface real estate and thus permit higher levels of functionality to be achieved. Such high index contrast OICs are often referred to as “high delta-n” waveguides, referring to a large (e.g., 2%-10%) difference between the core and cladding refractive indices in the OIC planar waveguides.
To achieve high packing density in optical circuits, the difference in refractive index between the core and the cladding should be increased so that the core size may be reduced. Accordingly, high delta-n waveguides allow for decreased core size and tighter turning radii at equal energy loss, and allow for less crosstalk for closely spaced waveguides due to better mode confinement in the core. In addition, since the core in a high delta-n waveguide may be thinner and the mode is more tightly confined, the thickness of the core and cladding layers used to make the planar waveguide of the OIC become thinner. This can reduce the costs and challenges in fabricating a high delta-n OIC, especially those made using traditional inorganic glasses that utilize etched cores.
Despite such potential benefits, high delta-n waveguides yet remain to be adopted due to a number of challenges. One of the greatest challenges preventing the commercial use of high delta-n waveguides is that high delta-n waveguides are not well suited for direct coupling to commonly used single mode optical fiber that typically has a 7 μm to 9 μm mode field diameter. The lack of compatibility between such components is understood in terms of optical mode theory.
Planar optical waveguides, including high delta-n waveguides, and optical fiber waveguides useful in high-speed and long-haul optical transmission systems often are designed to support a single mode. Stated differently, the waveguides are designed such that the wave equation has one discrete solution; although an infinite number of continuous solutions (propagation constants) may be had. The discrete solution is that of a confined mode, while the continuous solutions are those of radiation modes.
Because each waveguide will have a different discrete (eigenvalue) solution for its confined mode, it is fair to say that two disparate waveguides, such as an optical fiber and a planar waveguide, generally will not have the same solution for a single confined mode. As such, in order to improve the efficiency of the optical coupling, it is necessary to have a waveguide transition region between the planar waveguide of the OIC and the optical fiber. This transition region ideally enables adiabatic compression or expansion of the mode so that efficient coupling of the mode from one type of waveguide to another can be carried out.
As mentioned, optical fibers typically support mode sizes (electromagnetic field spatial distributions) that are much larger, both in the horizontal and vertical directions than modes supported by high delta-n waveguide structures, such as planar waveguides. Therefore, a challenge is to provide a waveguide transition region that enables adiabatic expansion of the mode so that it is supported by the optical fiber. Moreover, it is useful to achieve the adiabatic expansion of the mode in both the horizontal and vertical directions. Fabricating a waveguide to effect adiabatic expansion of the mode in the vertical direction has proven difficult using conventional fabrication techniques. For example, tapering the thickness of the waveguide to effect the vertical adiabatic expansion of the mode is exceedingly difficult by conventional techniques.
Consequently, there remains a need in the field for devices for effecting efficient coupling between waveguides having disparate characteristic modes, such as mode mismatch between high delta-n waveguides, (e.g., ridge lasers and silicon-on-insulator (SOI) rib waveguides), asymmetric mode devices, and prevailing (low) delta-n waveguides (e.g., single mode fiber).